Once, a nice swing and good putting could keep you on the PGA Tour, no matter how doughy you might be. Not anymore.

To understand how important physical fitness is to the highest level of professional golfers, know that the PGA Tour now brings its own gym-and-spa-on-wheels to each event.

Or, listen to Dustin Johnson — who at 6-4 can dunk a basketball — list the fittest players on the Tour: “Tiger [Woods] is very fit. [Martin] Kaymer, he works hard. Most of the guys at the top are all really hard-core into fitness.”

The golfer with the most major titles, Jack Nicklaus, once offhandedly was called “the fat kid” by older rival Arnold Palmer. Nicklaus slimmed down through the years, but the PGA Tour remained one of the few places in sports someone with an overflowing midsection such as Craig Stadler, The Walrus, could stand triumphant.

Now, golf’s dominant image could be that of Woods, answering questions Wednesday at the Trump Doral Golf Resort before the World Golf Championships-Cadillac Championship. His white golf shirt hanging off broad shoulders topped with sloping trapezius muscles and thick forearms that would do a wrist wrestler proud.

Woods was at the forefront of golf’s fitness boom. Just as Martina Navratilova, then Steffi Graf, then the Williams sisters repeatedly raised the athleticism bar in women’s tennis, so did the generation of golfers who grew up after high schools began implementing weight programs for all sports forced change.

In discussing the sore Achilles that prompted him to withdraw from the Cadillac Championship during the final round last year, Woods said, “I can now actually train instead of rehab. I’ve made some pretty significant gains in my strength. It feels nice to be able to train and not do the little bitty knick-knack rehab things.”

Asked what difference that has made in his game, Woods said, “That’s one reason I’m hitting it further. I have my legs underneath me, and that’s where our power is.”

Woods visits the VisionWorks Player Mobile Health and Fitness Trailers regularly to work out and even hang out for a while — the trailers can be a players-and-trainers only sanctuary. One trailer houses weights, kettlebells, medicine balls and treadmills and workout gear and the other contains massage tables and physical therapy equipment.

“The majority of players utilize one truck or the other depending on what’s going on [with them physically],” said Jason Stodelle, a physical therapist who works in the trailers. “This provides them with a sense of consistency. They might have a nice gym here [at Doral] but where we are next week maybe doesn’t. This gives them the opportunity to use the same equipment and staff week in, week out.”

Most people underestimate the stamina of playing golf while walking 18 holes. That’s 4.1 miles on Doral’s Blue Monster, the Cadillac Championship course.

Matt Kuchar recalls playing a Pro-Am round with a point guard for the NBA’s Memphis Grizzlies.

“He was an OK golfer, but he hated having to walk 18 holes,” Kuchar said. “He said, if he was going to use his local course and it was cart path only, he refused to play. He couldn’t handle all the walking from the cart and back. This is a guy that runs up and down the court all day long and it more fit than any of us and was struggling with the endurance of walking 18 holes.

“It seems silly, but there is certainly that endurance.”

That’s because it isn’t just walking. It’s walking and thinking and applying torque to your body. Coaches in every sport say it: Physical fatigue leads to mental fatigue, and as 2012 Masters champion Bubba Watson says, “Golf is the most mentally challenging sport there is.”

Watson, one of the tour’s longest hitters, joked about the invisibility of his biceps even when flexed. But nobody understands better than golfers the importance of strengthening the core, a major focus of personal trainers in the past decade.

“You work on certain areas, certain muscles so that it protects your knees, your elbows, your shoulders,” he said. “Once your core gets a little bit stronger, that’s going to help everything else. During my golf swing, I twist and turn a lot pretty rapidly, pretty violently, so it’s all about functional movements to help prevent the injuries as much as possible.”

Golfers know that no matter how many of them look like male models, their sport always will have the stigma of being not quite “athletic” as football, baseball, basketball or even tennis are.

Kuchar said, “As far as golf being athletic, I would say, ‘Go try it, let’s see how well you do.’ ”